I'm late to this convo but thought I'd chip in anyway.
I have an Ostrich bag on a Rawland Raidoverks rando rack. The Ostrich bag is held on by a VO decaleur. I love the set up and not had any problems. I have 40cm flared bars btw so no hand rubbing.
Without a decaleur I think that the restrap magetic thingy seems like your best option (I haven't tried one though) or strapping a basket to a rack with cable ties.
Ostrich do bags that have loops that strap to the handlebar but this looks like a bit of a faff to take on and off.